1. Field of the Invention
The present invention relates to an image forming system in which an image forming apparatus and optional printing-medium feeding device communicate with each other, and to the printing-medium feeding device.
2. Description of the Related Art
Optional units such as a printing-medium feeding device are connected to an image forming apparatus in order to extend various capabilities of the apparatus. Such an optional unit and the image forming apparatus are connected by a signal line for transmitting information. For example, if a plurality of optional units have been connected, it is necessary that the state of each optional unit be checked in order to feed or transport a printing medium. Whether or not a printing-medium feeding device contains a printing medium and whether or not a cassette has been loaded in the printing-medium feeding device are examples of detectable states of a printing-medium feeding device serving as an optional unit. It is required that a plurality of such states be checked. In a case where a plurality of optional units have been connected, therefore, it is required that the image forming apparatus send and receive information to and from each of the optional units efficiently.
As for a communication interface between an image forming apparatus and optional units, the specification of Japanese Patent Application Laid-Open No. 08-008935 discloses an image forming apparatus to which a plurality of optional units are connected successively by a bus. The image forming apparatus described in Japanese Patent Application Laid-Open No. 08-008935 is so adapted that prescribed processing is communicated to the plurality of optional units simultaneously and the optional units operate in conformity with the processing content. Further, the specification of Japanese Patent Application Laid-Open No. 04-199159 discloses an image forming apparatus provided with means for notifying a host apparatus of a change in the internal status of a lower-order device.
Although the number of times communication relating to an instruction from the main body of the image forming apparatus can be reduced with the image forming apparatus described in the specification of Japanese Patent Application Laid-Open No. 08-008935, it is required that response signals from the additional optional units be received individually. Consequently, in a case where the image forming apparatus monitors the state of paper transport in the overall system, it is necessary that communication be performed a number of times equivalent to the number of optional units that have been added on. The problem that results is an increase in communication time period and communication load.
In a case where the image forming apparatus described in the specification of Japanese Patent Application Laid-Open No. 04-199159 notifies a connected optional unit of internal information of the apparatus, a separate notifying signal line is required. Further, it is required that a signal transmitted from the image forming apparatus to the optional unit be transmitted separately. The problem that results is an increase in communication time period and communication load.
A specific example of a problem is that it is not possible to shorten the intervals at which paper feed is executed in a paper feeding device mounted as an optional unit. Normally, when printing paper that has been loaded inside a paper feeding device is fed, it is necessary to suppress the occurrence of jamming and unwanted wear of the feed rollers. Accordingly, the image forming apparatus instructs a certain paper feeding device to feed printing paper, thereby causing the device to execute the paper-feed operation. After positively sensing whether printing paper is still loaded in the printing device following the completion of the paper-feed operation, the image forming apparatus issues the next paper-feed instruction. In a case where time for communication between the image forming apparatus and a plurality of printing devices is great or in case of a large communication load, it becomes necessary to lengthen the paper-feed interval in order that this processing may be executed (this interval is referred to as the “time between sheets” below). If the time between sheets is lengthened, the throughput of the image forming operation (the number of printed sheets per unit time) declines.